1. Field of the Invention
The invention relates to the treatment, and particularly the encapsulation and deagglomeration of particles previously difficult to coat in a fluidized bed, for example, because of the size and/or density of the particles. Such particles generally comprise metal, binder, ceramic, refractory, or polymer with a coating or coatings of metal, binder, ceramic, sintering aid, or polymer.
2. Background of the Invention
Encapsulated powders have been produced for decades for a variety of purposes including fuel particles, pigments and paints, pharmaceuticals, magnetic components, fluorescent lamps, RF shielded plastic fillers, abrasive particles, metal matrix composites, and electronic applications.
Tough-Coated Hard Powders (TCHP) now promise to usher in a new family of materials exhibiting new paradigm combinations of properties including wear resistance, toughness, and light weight. In tool applications, it is expected that TCHP's will greatly surpass the performance of chemical vapor deposited (CVD)-coated carbide inserts. Since the powder contained within an insert has a surface area thousands of times greater than that of the insert itself, CVD-coating TCHP's (a surface-dependent process) is clearly more efficient. In addition, the thickness of coatings on micron-sized particles is several orders of magnitude thinner than the coatings on the outsides of inserts, reducing the time necessary to coat powder versus tool inserts. To achieve the full potential TCHP's will require the development of an industrial scale process for coating ceramic or metal powders, especially particles less than approximately 10 microns in size and lighter than about 10 g/cc, hereafter referred to as “Geldart Class C” particles.
Many different methods have been used to produce these coated powders, including spray drying, electroless plating, molten salt, plasma coating, chemical vapor deposition, physical vapor deposition, and others. One of the most successful and well-known methods has been CVD coating, particularly using fluidized bed technology.
Much is taught in the prior art about preparation of coatings on powders. As the need for smaller and smaller coated particles increases, the principle barrier to achieving uniform, contiguous coating has been the tendency for particles below 10 to 15 microns to severely agglomerate and clump. Ordinary horizontal fluidized beds and barrel coaters are simply unable to overcome the strong interparticulate attractors, such as van der Waals forces, that increase with decreasing particle diameters. Accordingly; a variety of apparatus has been developed in an attempt to overcome these deficiencies.
Few of the prior art methods and apparatus are capable of effectively coating contiguous and homogeneous coatings on Geldart Class C powders. One exception is platelet-shaped particles, where the small thickness-diameter particle aspect ratio is calculated as within Geldart Class C but the large 15-25 micron diameter allows it to fluidize as if it were a much larger average particle size.
One of the most widely-used methods of coating powders is the fluidized bed. Fluidized bed reactors are especially useful and economical to operate because of the high exposure of solid surfaces to the carrier and decomposing coating source reactant gases. This benefit, however, is considerably offset by the principal limitations of fluidized beds: agglomeration and bonding of the powders.
As is well-known, for example, when fluidized beds are used for the production or coating of very fine powders, particles in the bed are susceptible to (a) agglomeration by van der Waals and other interparticulate attractor forces and/or (b) sintering or cementing together of particles by the coating being deposited on their surfaces at high temperature. If these agglomerated or sintered particles are not continuously broken apart, their points of contact prevent complete coating and the lumps may tend to grow and settle to the bottom of the fluidizing bed, greatly reducing effective surface areas.
In a powder coating process, the key particle parameters are particle density, size, and temperature. It is well understood that particles of low density are much more easily entrained by fluidizing gases than are heavier particles. Further, from Geldart's classifications of particles (see Kunii and Levenspiel, Fluidization Engineering, 2nd ed., Butterworth-Heinemann, 1991, pp. 75-79), it is known that agglomeration or sintering increases in fluidizing Geldart Group C fine particles as the particle size decreases below about 40 microns. In “Types of Gas Fluidization,” Power Technology, 1973, pp. 285-292, D. Geldart classifies the behavior of fluidized beds into groups depending on the density difference between the fluidizing gas and the solids and mean particle size. Group A powders exhibit dense phase expansion after minimum fluidization and prior to incipient bubbling operation. Group B powders bubble at the minimum fluidization velocity, and Group C (below approximately 10 microns and lighter particles below about 10 g/cc) are difficult to fluidize at all, and those in group D can form stable spouted beds. When fluidizing Geldart Group B or D particles (larger than about 100 microns), interparticle cohesive forces are negligible compared with the forces the fluidizing gases exert on the particles.
It is also known (e.g., in Ceramic Fabrication Processes, ed. W. D. Kingery, MIT Press, Cambridge, Mass., 1958, pp. 120-131) that the probability for particles to bridge and adhere to each other increases with increasing temperature and with increasing duration of the interparticulate contact. Because most coated powder applications require complete and homogeneous coating layers, the foregoing factors prevent the use of all but a small number of the methods and apparatus taught in the prior art.
One approach to deagglomerating fine powders during their coating is embodied in the Recirculating Fast Fluidized Bed Chemical Vapor Deposition Reactor (RFFB-CVDR), disclosed in U.S. Pat. No. 5,876,793 to Sherman, et al. In the RFFB-CVDR, the individual particles are fully coated by recirculating them continuously in a recirculating fluidized bed operated in fast-fluidizing or turbulent fluidization conditions, with no distinct boundary between the top of the bed and the vapor phase above the bed. Each time an individual particle passes through the reaction zone, it receives a few more angstroms of coating because the particles are deagglomerated in a turbulent gas stream that individualizes the particles by high-shear forces and interparticulate collisions that break up the agglomerates. This allows complete exposure of the particle surface to the coating vapors. To prevent agglomeration and to obtain uniform coatings, it is necessary to operate the system well-above the bubbling conditions characteristic of most fluidized beds. The velocity of the gas stream in the fluidized bed is high, approximately 0.1 to 1.0 m/s. At these gas velocities, the particles, unless relatively large or relatively heavy, are carried through the bed before achieving their desired thickness of coating. Recirculating the particles is then necessary. Because the velocity of the gas stream in the reaction zone is such that it quickly carries substantially all of the particle upwardly out of the bed, many passes through the reaction zone are generally necessary to adequately coat a particle. The particles, except for agglomerates, are generally not classified in the primary reaction zone because the bed is operated at too high a gas flow rate. As each particle makes one pass through the system, it accumulates only a small amount of coating thickness, requiring multiple passes through the reaction zone to build up the desired thickness.
Because of the high gas flow velocities, submicron light density (2-9 g/cc) particles and intermediate to heavy density (10-20 g/cc) particles and below about 5 to 10 microns are strongly entrained by the turbulent gas flow and must be recirculated as many times as is required to build up the desired coating thickness. These particles must be separated from the gas stream by one or more cyclones or by other gas-solid separation and filtration methods well-known in dust collection processes, and collected in a second bed which may or may not be fluidized, downstream of the reaction zone. When the first bed is operated in the fast-fluidization condition, the velocity of the gas stream is in excess of the terminal velocity of the particle in that stream and the particles are quickly carried into the reaction zone and into the collection zone. After collection, the particles must be transported from the collection zone in the second bed to the primary fluidized bed by mechanical, pneumatic, or other means. The second bed, if fluidized, is operated at gas velocities well below turbulent conditions.
Even such reactors, capable of fluidizing, deagglomerating, and coating uniform Geldart Group C particles, are challenged beyond their capabilities of coating contiguous and homogeneous coatings in many practical industrial situations.
For example, an important limitation of the RFFB-CVDR and other high gas-shear reactors is that the gas-stream deagglomeration principle fundamentally requires high gas stream velocities. Such velocities are required to entrain and elutriate a very high percentage of the distribution of the particles being coated to allow them to be captured and collected by various means in order to recirculate and recoat them. Elutriation is the process in which fine particles are carried out of a fluidized bed due to the fluid flow rate passing through the bed. Typically, fine particles are elutriated out of a bed when the superficial velocity through the bed exceeds the terminal velocity of the fines in the bed. However, elutriation can also occur at slower velocities.
Fine particles in fluidized beds come from feed streams; commercial core powder mechanical milling, attrition, or breakage of larger particles. Temperature or diffusion stress cracking and size reduction due to chemical reactions and shrinkage can also result in fine particles. When fines elutriation is a problem that cannot be reduced or eliminated with modifications to the bed design, fines can often be recovered such as with cyclones or hydrocyclones. Leva (Chem. Engr. Prog., 47, 39, 1951) measured the rate of elutriation (total mass per time) from a bed of particles with a bimodal size distribution. He found that:                (1) When the column height above the bed is small, the elutriation rate is high. However, if the column height exceeds a certain minimum size then the rate is constant minimum value. This occurs because small particles that are expelled from the top of the bed have high velocities that require greater distance to slow down and turn around to return to the bed.        (2) The elutriation process causes a decrease in particle concentration. A fluidized bed behaves similar to a mixture of liquids with different volatilities. For example, in the liquid-liquid mixture, the more volatile material leaves the mixture at the lower boiling temperature. By analogy, the finer particles have a lower boiling temperature than the larger particles. The boiling temperature is analogous to the fluidization velocity. The higher the velocity, the greater the rate at which the low boilers will leave the bed. The free space height above the bed serves as a condenser, to cool and slow down the elutriated particles and return them to the mixture. The greater the boiling rate, the greater capacity that is needed of the condenser, hence the greater free space height.        
In operating the fluidizing gases to levitate and recycle the mid to large size particles in the primary reaction zone to prevent them from settling, uncoated, in the bottom of the bed, the smaller particles are elutriated with the high volume and high speed gas flow stream and must be recycled.
The elutriation phenomenon leads to a major limitation of the RFFB-CVDR. Most commercially-available powders have a wide particle size distribution that is often bi- or tri-modal. In smaller Geldart Class C powders, such a distribution will often have a substantial percentage (15-25 percent of particles) of nanofines. Thus, heavy powder particles averaging below about 0.5 microns and light density particles averaging below about 5 to 10 microns contain major percentages of particles that are extremely difficult to separate from gas streams which overload the particulate collection systems and rapidly clog them. The particle size described throughout the specification is based on a number count distribution.
When it is a primary objective to coat smaller Geldart Class C powders, as in the case of Tough-Coated Hard Powders (TCHP), operating this equipment was traditionally difficult to impossible. It has been demonstrated using the RFFB-CVDR equipment on commercially-available 2-micron titanium nitride core powders, that nanofines quickly overload and clog the dust collection and filtration system. Avoiding this condition requires an expensive and quality-degrading (oxygen-inducing) Stokes Law sedimentation classification process to separate the fines and substantially reducing the yield of the incoming core powders.
Another major disadvantage is that the RFFB-CVDR, which operates in turbulent gas flow fluidization conditions, cannot be scaled down to smaller-diameter, smaller-capacity fluidized beds for research and development and for high-value products. The reason is that with smaller diameter RFFB-CVDR reactors, the fluidization gas flow becomes laminar, severely reducing both the interparticulate collisions and the high gas shear forces that break up agglomerates. The smallest RFFB-CVDR reactors have large fluidized bed diameters that require at least 10 to 15 kgs of light and small core particles, resulting in a lot weight of at least 50 to 60 kgs of material. Test increments of different coating weight percentages of the intermediate and binder coatings, plus test increments of extremely expensive core powder such as cubic boron nitride or diamond, or test increments involving different carbon percentages all become practically and economically out of reach because milling in test increments breaks off the coatings and blending has been found to be highly ineffective at these grain sizes.
Yet another major disadvantage of the RFFB-CVDR occurs when it is used to coat small and/or very light density Geldart Class C powders with 50-75 wt % of tungsten, for example. Here, the fluidization gas and thermodynamic parameters must be continuously altered as the particles gain size and weight. This requires sophisticated modeling, monitoring, instrumentation, and control systems to achieve industrial process and product repeatability.
Yet another major disadvantage of the RFFB-CVDR stems from the reduced residence times spent in the coating zone by the smaller particles and nanofines in a Geldart Class C particle size distribution. The RFFB-CVDR, by its inherent function, recycles high volume fractions of particles for long waits in the return system, while the heavier particles tend to reside full-time in the reaction zone, increasing coating thickness disproportionately. It has been found that this reduced uniformity of the coating thickness on both large and small particles in the distribution has a potentially severe effect. Longer residence times in the collection zones decreases the coating thickness of, for example WC on TiN core nanoparticles, while thinner WC coatings increase the probability that nitride and carbide core particles will be dissolved by and interact chemically with the other coatings such as cobalt or tungsten carbide during sintering into useful articles. Such interactions are characteristic of cermets and significantly reduce fracture toughness and wear resistance of the finished article. This process, involves significant heat losses, and thus reduces the thermal efficiency of the RFFB-CVDR reactor.
Still another major disadvantage of the RFFB-CVDR is that as the smaller particles are separated, collected, and recirculated into the reaction zone, they cool and must be reheated, wasting the heat energy gained in the reaction zone. Finally, this long residence time of the nanoparticles outside the hot reaction zone varies according to the amount of powder in the lot, creating two other major disadvantages. The larger the volume of powder in the return hopper, the less coating the smaller particles receive, creating a quality variation with lot size. The thinner the coatings of these nanofines, the higher the probability that these nano-coatings in TCHP will be breached during liquid phase sintering, allowing the cobalt to react with the core particles and weaken the WC substrate.
A well-known alternative approach to contiguously coating small and/or very light density Geldart Class C powders is to suspend the particles in a liquid medium. Here, the agglomerative attractor forces may be reduced by capillary and surface tension forces, and the shear forces that can be applied to the agglomerates that do form are orders of magnitude larger than with gaseous media. Examples of coating processes using liquid media are electroless, electrolytic, spray-dry, and non-aqueous solution-based chemical or electrolytic processes. The key barriers to wider use of these methods include removal of the liquid media after coating, residual salts or other products, oxidation, and coagulation of the coated powders.
If Geldart Group C core particles were commercially available in uniform, mono-modal, and/or narrow particle size distribution, this would considerably alleviate the limitations of the RFFB-CVDR and the use of liquid media. In fact, some different particle creation methods have resulted in commercial availability of a very few uniform, mono-modal, and/or narrow particle size distribution powders. This may become a trend with increasing numbers of core particle materials, but producing such uniform powders of many different ceramic materials is in itself a highly challenging scientific endeavor.